The present invention relates to an A/D (Analog to Digital) converter, which converts an analog signal into a digital signal.
Two types of typical A/D converters, successive approximation A/D converter and pipelined A/D converter, have been proposed and used.
According to a conventional successive approximation A/D converter, it is necessary to spend at least N cycles to obtain N-bit digital data. Therefore, it is difficult to provide a fast operating A/D converter.
According to a conventional pipelined A/D converter, a large number of analog circuits must be used, so that it is required to increase a dimension of each transistor in order to provide higher accuracy. As a result, the scale of circuitry becomes larger.
Accordingly, it is an object of the present invention to provide an A/D converter which operates at a high speed as compared with a conventional successive approximation A/D converter.
Another object of the present invention is to provide an A/D converter which can be designed to have a smaller scale of circuitry.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to a first aspect of the present invention, an A/D converter includes first to Nth stages of A/D conversion units, which are connected in series, each A/D conversion unit converting an analog input signal into a digital output signal. Each of the A/D conversion units includes a) a sample-and-hold circuit, which holds an analog input signal; b) a selector which selects one from a plurality of reference voltage signals in accordance with a digital output signal outputted from the one stage preceding A/D conversion unit; and c) a comparator which compares an output signal supplied from the sample-and-hold circuit with the reference voltage signal selected by the selector.
According to a second aspect of the present invention, the A/D converter according to the first aspect further includes a flash A/D conversion unit of m-bit flash type, where xe2x80x9cmxe2x80x9d is smaller than xe2x80x9cNxe2x80x9d. The flash A/D conversion unit comprises a m-bit flash A/D conversion circuit; a sample and hold circuit which holds an analog input signal; and m*(Nxe2x88x92m) pieces of flip-flop circuits, in which each of serially connected (Nxe2x88x92m) pieces of flip-flop circuits form a line so that xe2x80x9cmxe2x80x9d lines of (Nxe2x88x92m) flip-flop circuits are connected in parallel to output terminals of the m-bit flash A/D conversion circuit.